Add executable flag to sbox copy requests

Add a boolean flag to the sbox proto to request the executable bit
to be set after copying a file.  This will be used for sandboxing
tools when copying a file in from the source tree that would
normally get its executable bit set during installation.

Bug: 124313442
Test: sbox_test.go
Change-Id: Ie2c197bb5183ffc1bf63fd6effd175143cd324d4